Aluminum alloy

ABSTRACT

This disclosure teaches a novel aluminum alloy having high resistance to oxidation during hot rolling, improved surface appearance and improved bright anodizing characteristics and the method of processing same. The alloy contains from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balance essentially aluminum, wherein the silver is substantially dissolved in solid solution in the matrix.

11M es Patent [191 Brock et a1.

[ ALUMINUM ALLOY [75] Inventors: Andrew Brock, Cheshire; Michael J.Pryor, Woodbridge, both of Conn.

[73] Assignee: Olin Corporation, New Haven,

Conn.

22 Filed: Oct. 24, 1972 21 Appl. No.: 299,917

Related US. Application Data [62] Division of Ser. No. 148,582, June 1,1971, Pat. No.

3,306,717 2/1967 Lindstrand et al 75/147 Primary Examiner-W. W. StallardAttorney, Agent, or Firm-Robert l-l. Bachman 57 ABSTRACT This disclosureteaches a novel aluminum alloy having high resistance to oxidationduring hot rolling, improved surface appearance and improved brightanodizing characteristics and the method of processing same. The alloycontains from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percentsilver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percentsilicon, balance essentially aluminum, wherein the silver issubstantially dissolved in solid solutionin the matrix.

5 Claims, 2 Drawing Figures 1 ALUMINUM ALLOY BACKGROUND OF THE INVENTIONAlloys containing from 0.5 to 3 percent magnesium with small amounts ofiron and silicon find extensive use commercially, for example,particularly as bright anodized automotive trim. They are generallyfabricated to sheet, bright dipped, anodizedin sulphuric acid and sealedso asto expose a bright reflective corrosion resistant surface fordecorative and functional purpose In automotive applications thesebright anodized aluminum alloys have appearance somewhat similar tobuffed stainless steels or to chrome-plated brass but are much moreeconomical to the user.

One of the problems in processing aluminummagnesium alloys for brightanodizing is the necessity of retaining magnesium-suicide in solutionduring the hot rolling process. This is mandatory in order to obtainproper response to bright dipping and to sulphuric acid anodizing sothat excellent specular reflectivity is obtained. Retaininginagnesium-silicide in solution during hot rolling involves initiatingthe rolling at unusually high temperature in, therange of 850 to'9 75Fdepending upon the magnesium content of thealloy. At these hightemperatures, the oxidation resistanceof aluminum-magnesium alloysdeteriorates rapidly and copious amounts of loosely adherent magnesiumoxide are formed as part of the high temperature oxidation productduring hot rolling. The magnesium oxide can either adhere to the rollsor can be ground into the soft metal surface. Either eventuality givesrise to surface defects known generically as hot mill pickup. Thispickup is never completely removed during subsequent processing andresultsin defects in the final bright anodized sheet that tend todetract from specular reflectivity and from subsequent corrosionresistance.

Accordingly, it is a principal object of the present invention toimprove the high temperature oxidation resistance of aluminum-magnesiumalloys intended for use in the bright anodized condition. I

It is an additional object of the present invention to provide novelaluminum alloys having a superiorresponse to bright dipping and a methodfor processin same which obtains improved properties.

It is afurther object of the present invention to provide novel aluminumalloys which have improved reflectivity after bright anodizing.

Further objects and advantages of the present invention will appear fromthe ensuing specification.

SUMMARY OF THE INVENTION In accordance with the present invention, theforegoing objects can be readily achieved.

The improved alloys of the present invention consist essentially of from0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balanceessentially aluminum, wherein the silver is substantially dissolved insolid solution in the matrix.

The alloys of the present invention are characterized by many desirableadvantages, The alloys have markedly improved resistance to oxidation inthe temperature range of 850 to 975 F which results in improved surfaceappearance after hot rolling. The alloys are tolerant to a broader rangeof solution composition in which they can be bright dipped. The alloysfurther exhibit substantially improved brightness after anodizing insulphuric acid and sealing.

The present invention also provides a process for obtaining improved,bright anodizing alloys which comprises:

A. providing an alloy consisting essentially of from 0.5 to 3 percentmagnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percentiron, from 0.001 to 0.15 percent silicon, balance essentially aluminum;I

B. holding said alloy for at least 15 minutes within the temperaturerange of 850 to 975 F; v

C. hot rolling said alloy with a starting temperature I within saidholding temperature range;

D. cold rolling said alloy; and E. heating said alloy within thetemperature range of 350 to 750 F for from 5 seconds to 8 hours.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which form a part ofthe present specification:

FIG. 1 is a graph of weight gain versus time when a comparative Alloy Ais compared with Alloy B, an alloy of the present invention. The graphand results are discussed in the examples.

DETAILED DESCRIPTION OF THE INVENTION As indicated herein and above, thealloys of the present invention are characterized by greatly improvedresistance to oxidation within the temperature range of 850? to 975 Fand exhibit a much superior hot rolled surface and specular reflectivityafter bright dipping. These improved surface characteristics areobtained in accordance with the alloys and process of the presentinvention. In addition, the alloys of the present invention have goodmechanical properties and workability.

The favorable surface characteristics achieved by the alloys of thepresent invention result in greatly reduced product rejections due tohot mill pickup during hot rolling at high temperature. The oxidationproblem cannot be solved in conventional alloys by reducing hot rollingtemperature and thereby increasing oxidation resistance becausemagnesium-silicide now forms in the microstructure of the alloy anddetracts from. the response to bright dipping and bright anodizing.

The alloys of the present invention do not completely inhibit theformation of magnesium oxide as a reaction product during heating in thetemperature range from 850 to 975 F. However, the amounts of magnesiumoxide formed are tremendously reduced and it appears to exist as acompact transparent film that is not removed from the metal during hotrolling. It is a surprising feature of the present invention that therange of acid concentrations over which excellent brightening can beobtained in bright'dipping is substantially increased with the alloys ofthe present invention. Accordingly, the incidence of unacceptableproducts in subsequent bright dipping and anodizing is significantlyreduced. It has been found that due to the foregoing and due to theimproved rolled surface, loss of brightness during anodizing insulphuric acid is dramatically reduced.

As indicated hereinabove, the alloys contain from 0.5 to 3 percentmagnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percentiron, from 0.001 to 0.15 percent silicon, balance essentially aluminum.It is preferred that the magnesium content be from 0.8 to 2.8 percentand that the silver be from 0.02 to 0.1 percent. The alloy may alsocontain one or more of the following: manganese in an amount up to 0.10percent, copper in an amount up to 0.10 percent, zinc in an amount up toO. percent, chromium in an amount up to 0.05 percent, and titanium in anamount up to 0.05 percent. Naturally, amounts as low as 0.001 percent ofthe foregoing may be present.

The process of the present invention is briefly described hereinabove.The alloy is held within the temperature range 850 to 975 F for at leastminutes, with the maximum holding time not being critical. Following theholding step, the alloy is hot rolled, with the starting temperaturebeing within said temperature range and with a preferred reduction of atleast 70 percent. The alloy may be cooled from hot rolling by anydesired means, such as by water quenching. The alloy is then coldrolled, with a preferred reduction of at least 50 percent followed byheating for from 5 seconds to 8 hours within the temperature range of350 to 750 F.

As indicated hereinabove, the resultant material has been found to havemany, highly desirable characteristics, such as improved response tobright dipping and bright anodizing and reduced oxidation rate.

The present invention will be more readily apparent from a considerationof the following examples:

EXAMPLE I A comparative Alloy A, containing 2.4 percent magnesium, 0.036percent iron, 0.044 percent silicon, 0.041 percent copper, 0.0088percent titanium, balance essentially aluminum, was cast as a directchill ingot measuring 3 inches X 7 inches X 42 inches. 1/8 inch wasmilled off each face of the ingot. The ingot was preheated to 925 F andheld at temperature for a period of 8 hours followed by rolling onpolished steel rolls with two successive reductions in thickness of and30 percent. A substantial amount of oxide was transferred to thepolished steel rolls and the surface of the hot rolled alloy was roughand discontinuous.

EXAMPLE II An alloy, Alloy B, containing 2.5 percent magnesium, 0.05percent silver, 0.036 percent iron, 0.043 percent silicon, 0.045 percentcopper, 0.0089 percent titanium, balance essentially aluminum, wasdirect chill cast, scalped and hot rolled in an identical fashion tothat described in Example I. The polished steel rolls were essentiallyfree from oxide transferred from the hot aluminum alloy and the hotrolled surface was uniform and free from defects.

EXAMPLE III Alloys A and B of Examples I and II were heated to 925 F,held for 8 hours and hot rolled in 10 passes to 6 a final thickness of0.1 inches. The temperature after 7 the last pass was 700 F and thealloys were then quenched in still water. The alloys were subsequentlycold rolled to a thickness of 0.030 inch and were partially annealed at500 F for 2.5 hours. After processing, the surface of Alloy A showedextensive hot mill pickup of magnesium oxide. On the other hand, thealloy of the present invention, Alloy B, showed excellent surfacesmoothness and was substantially free from hot mill pickup.

EXAMPLE IV Alloys A and B, processed in accordance with Example III,were heated to 932 F in air and held at that temperature for 24 hoursduring which time continuous recording of the oxidation weight gain weremade with a recording microbalance. The results are shown in FIG. 1. Thegraph clearly shows that Alloy B, the alloy of the present invention,had a much reduced oxidation rate and showed essentially no oxidationbetween times of around 4 and 12 hours. By contrast, the comparativeAlloy A showed essentially a high and constant rate of oxidation duringthat period, with the weight gain curve being essentially linear.

EXAMPLE V Alloys A and B, processed in accordance with Example III, wereimmersed for 120 seconds in a solution containing percent phosphoricacid and various concentrations of an aqueous, 70 percent nitric acidsolution with the specimen potentials being also measured. Thetemperature of the bath was 83 C. The results, summarized in FIG. 2,show that acceptable bright dipping was obtained for Alloy B, the alloyof the present invention, over a wider range of nitric acidconcentrations than the comparative Alloy A. The reflectivity of AlloyB, the alloy of the present invention, after bright dipping was 79percent with a silver mirror being used as a standard of percentreflectivity. On the other hand, Alloy A had a reflectivity of only 58percent after bright dipping.

EXAMPLE VI The bright dipped specimens of Example V were anodized for 20minutes in 15 percent sulphuric acid at 25 C with a current density of20 amps per square foot. After anodizing the alloys were rinsed and weresealed for 10 minutes in boiling water containing 3 ppm phosphate addedas Na i-IP0 The reflectivity of Alloy B after anodizing and sealing was44 percent; whereas, the reflectivity of Alloy A was only 34 percent,with a silver mirror standard being rated at 90 percent in both cases.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

l. A process for obtaining an improved bright anodized alloy comprising:

A. providing an alloy consisting essentially of from 0.5 to 3 percentmagnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percentiron, from 0.001 to 0.15 percent silicon, balance essentially aluminum;

3. A process according to claim 1 wherein said cold rolling is with areduction in thickness of at least 50 percent.

4. A process according to claim 1 wherein said alloy is quenched afterhot rolling.

5. A process according to claim 1 wherein the resultant material isheated in air at elevated temperature whereby reduced oxidation rate isobtained.

2. A process according to claim 1 wherein said hot rolling is with a reduction in thickness of at least 70 percent.
 3. A process according to claim 1 wherein said cold rolling is with a reduction in thickness of at least 50 percent.
 4. A process according to claim 1 wherein said alloy is quenched after hot rolling.
 5. A process according to claim 1 wherein the resultant material is heated in air at elevated temperature whereby reduced oxidation rate is obtained. 